1. Field of the Invention
The present invention relates to an optical fiber type wavelength converter using Cerenkov radiation phase matching.
2. Description of the Prior Art
Wavelength converters have been actively developed, which use nonlinear optical crystal to constitute an optical waveguide passage to guide an optical wave to a minute area, and effectively generate a second harmonics. These wavelength converters are generally classified into two types depending on the methods satisfying phase matching. The first type matches a nonlinear polarization wave excited by the incident light with the phase velocity of the second harmonics wave and executes the phase matching between the guide mode of the fundamental wave, i.e., the incident light and the guide mode of the second harmonics. The other type executes so-called Cerenkov radiation phase matching, i.e., the phase matching between the guide mode of the fundamental wave and the radiation mode of the second harmonics.
An optical wavelength converter is known, which is constituted in the shape of an optical fiber comprising a core of nonlinear optical crystal and a clad surrounding this core and employs the Cerenkov radiation phase matching. This optical wavelength converter is also known as an optical fiber type second harmonics generator (hereinafter referred to as "SHG"). According to the Cerenkov radiation system, it is possible to generate a second harmonics (hereinafter referred to as "SH") whose optical phase is almost automatically matched. The SHG is applied to a shortwave light generator.
As shown in FIG. 1, the shortwave light generator comprises a semiconductor laser 1, a coupling lens 2, an SHG 3 of which core is constituted by nonlinear optical crystal, and an axicon 4. The coupling lens 2 collects and guides the light emitted from the semiconductor laser onto the end face of the SHG 3. The axicon 4 shapes the wavefront of the SH wave, irradiated after conversion done by the optical wavelength converter, to form the SH wave in the form of the parallel flux of light.
FIG. 2 is a conceptual diagram of the SHG 3, which comprises a columnar core 10 and a cylindrical clad layer concentrically surrounding the core 10.
Referring to FIG. 2, when the core 10 whose fundamental wave mode has an effective refractive index N (.omega.) propagates through the core 10 from the left to the right in the diagram, the nonlinear polarization wave which generates an SH wave also propagates at the same phase velocity C/N (.omega.) (C: light speed). Suppose that this nonlinear polarization wave generates the SH wave in the direction that forms an angle of .theta. with the waveguide direction at the illustrated point A, and after a unit time, the polarization wave likewise regenerates the SH wave in the direction of 8 at the point B. If the SH wave generated at the point A propagates through, for example, the clad layer 20 and reaches the point C after a unit time, and .theta. is an angle to make the line AC intersect the line BC at the right angles, the wavefront of the SH wave generated between A and B by the nonlinear polarization wave becomes BC. This means that a coherent SH wave has been generated.
The SH wave generated in this manner propagates in clad mode in which it repeats the full reflection at the boundary between the clad layer 20 and air as shown in FIG. 3, and is emitted from the end of the fiber in a corn shape in the direction determined by .alpha.. The equiphase wave surface of the wavefront of the thus emitted SH wave is conical with the center axis of the fiber as its axis.
With the shortwave light generator module constituted by the above SHG, it is not desirable that the nonlinear optical crystal used for the core absorbs the light having half the wavelength of the semiconductor laser. However, there is no material of this type found yet which has a large nonlinear polarization constant; such a material has a nonlinear polarization constant of about 100 Pm/V at maximum. Further, the level of the output of the semiconductor laser used for the primary light is as high as 50 to 60 mW, and the coupling efficiency of the light to be coupled by the coupling lens is as high as 40 to 50%. Consequently, the wavelength converting efficiency (power of an SH wave to be emitted/output of the semiconductor laser) is about 0.1% at maximum.